This invention relates to a process for ionizing substances including aerosols, vapors and gases and analyzing the ionized substances by mass spectrometry.
Within the past few years there has been added to the electron impact ionization (E.I.) mode of ionizing substances in the gaseous state for mass spectrometric analysis a mode of ionizing substances known as chemical ionization (C.I.). In electron impact ionization the substance under investigation is bombarded with electrons, whereas in chemical ionization the substance is reacted with a known preselected set of reactant ions. In practice, chemical ionization is achieved by forming primary ions by electron impact ionization of a reactant gas at pressures greatly in excess of those used in electron impact ionization of substances (e.g. about 1 Torr for C.I. vs. about 10.sup.-.sup.4 Torr or less for E.I.). The primary ions thereby formed undergo ion-molecule reactions with electrically neutral reactant gas molecules so as to produce reactant ions. Finally, chemical ionization of the substance under investigation occurs in the same reaction zone by ion-molecule reactions of the substance with the reactant ions. Of course, all of these three steps occur substantially simultaneously and continually in the C.I. zone. In general, chemical ionization mass spectra have fewer ions and more intense high mass ions as compared to electron impact mass spectra and the two ionization techniques are highly desirable complementary analytical tools. A comparison and further analysis of these techniques may be found in applicant's article, "Chemical Ionization Mass Spectrometry," appearing as chapter 31 of "Biochemical Application of Mass Spectrometry," edited by G. R. Waller, Wiley-Interscience Publisher's, New York, New York, 1972.
Because of the complementary nature of the two ionization techniques, attempts have been made to provide a single ion source capable of both chemical and electron impact ionization at consecutive times, with the reactant gas pressure being the determinant of whether the sources operated in a C.I. or an E.I. mode. However, because the C. I. pressures are considerably above the typical pressure in the analyzer region of the mass spectrometer (usually less than 10.sup.-.sup.5 Torr), a gas tight region in communication with an ion source must be provided. This requires that the electron entrance and ion exit apertures be very small which in turn reduces significantly (by roughly an order of magnitude) the sensitivity of the source in the electron impact mode of operation, simply due to the decrease in the number of electrons entering the source through the narrow electron entrance aperture and the decrease of the number of ions leaving the source through the narrow ion exit aperture.
Moreover, when the ion source is fed from the outlet of a gas chromatographic column, an analytical technique gaining much importance wherein, the mass spectrometer becomes the detector for gas chromatography, additional problems for the combined C.I. - E.I. source are encountered. For chemical ionization, the technique is simple since the effluent from the gas chromatographic column may be fed directly to the ion source of the mass spectrometer, with the carrier gas of the effluent serving as the reactant gas in the ion source. However, if the effluent is to be subjected instead to electron-impact ionization, a splitter or a separator must be provided at the gas chromatography-mass spectrometer interface, thereby complicating an otherwise simple interface. Alternatively, the effluent flow rate is reduced drastically to maintain the pressure in the ion source at the level required for electron impact ionization. Such flow rate changes make difficult correlation between the C. I. and E. I. spectra of the effluent. Moreover, because of the large disparity in pressure required for the E. I. and C. I. modes, switch over in the middle of a gas chromatographic peak (typically of 15 seconds duration at 1/2 height) is not possible. Thus, distinct time-separated runs are needed on duplicate sample injections and hence, despite all efforts at identity and ignoring the considerable time loss in such duplicate injections, a great deal of effort is required to correlate the E. I. and C. I. spectra.
The invention described in the above-identified application provides a process and apparatus which eliminates the problems described above and provides a means for successfully subjecting a gaseous substance contemporaneously to electron impact and chemical ionization. The apparatus comprises two separate ionization zones wherein a gaseous substance first is subjected to chemical ionization at chemical ionization pressures and subsequently subjected to electron impact ionization at electron impact ionization pressures. Suitable pressures in each ionization zone are maintained by an aperture of appropriate size between the two ionization zones. Both electrically neutral and ionized gases are passed sequentially through the two ionization zones and subsequently to a mass spectrometer. The apparatus disclosed in the above-identified application can be used in the process of this invention which is described below.
In prior art processes involving chemical ionization, the reactant gas is chosen so that it permits a substantial portion of the substance being analyzed to become ionized but does not cause the substance to disassociate by virtue of a large energy transfer from the ionized reactant gas to the substance. However, when analyzing a substance by concomitant E. I. and C. I., the use of a single reactant gas incurs disadvantage which reduces the sensitivity of the total analytical procedure. For example, in present processes involving E. I., the use of high concentrations of methane gas as the reactant gas substantially reduces the sensitivity of the subsequently-employed mass spectrometer. This is because methane has a large ionization cross-section of 4.7 .times.10.sup.-.sup.16 cm.sup.2 and thus, when employed in high concentrations, is able to capture large portions of the electrons generated in the ion source in preference to the substance. On the other hand, methane as a reactant gas is desirable in C.I. since it has the ability to capture electrons, thereby forming reactant ions which ionize the substance to form relatively stable ionized substances. As a further example, helium has not been used as a reactant gas in C.I. since ionized helium would give rise to high energy transfer to the substance being analyzed. This energy transfer would not give rise to protonated molecules of the substance but would cause the substance to dissociate to give electron impact-like spectra. While this is desirable in E.I., it is undesirable in C.I. since the resultant spectra for both the C.I. and E.I. would be very similar and the advantages obtained by the use of the two complementary analytical techniques would be lost.